He made a similar appeal in a video posted on YouTube earlier this month.

"We appeal to whoever is caring for her to show compassion and allow Caity, Josh and our unborn grandbaby to come home," he said.

He suggested that his daughter and her husband may have been trying to help Afghans by joining an aid group after touring the region.

He described his daughter as "naive" and "adventuresome" with a humanitarian bent.

In his last email in October, Josh did not give their exact location but said they were not in a safe place.

The last withdrawals from the couple's account were made on October 8 and 9 in Kabul but there has been no activity since then.

"He just said they were heading into the mountains - wherever that was, I don't know," Mr Coleman said. "They're both kind of naive, always have been in my view.

"Why they actually went to Afghanistan, I'm not sure. I assume it was more of the same, getting to know the local people, if they could find an NGO (non-governmental organisation) or someone they could work with in a little way."

The US State Department and Canadian Foreign Affairs Ministry are both looking into the disappearance.

Canadian Foreign Affairs Ministry spokeswoman Chrystiane Roy called it a "possible kidnap" and said it was "pursuing all appropriate channels".

It was not known whether the silence over the case by US and Canadian officials and, until now, by the Coleman family was because of on-going negotiations to seek their release.

Information blackouts have kept some similar past cases quiet in an attempt to not further endanger those missing.

According to Hazrat Janan, the head of the provincial council in Afghanistan's Wardak province, the two were abducted in Wardak in an area about 25 miles (40 km) west of the capital Kabul.

They were passing through Wardak while travelling from Ghazni province south of Kabul to the capital.

Wardak province, despite its proximity to Kabul, is a rugged, mountainous haven for the Taliban and dangerous for foreigners travelling without military escorts.

Mr Janan said it was suspected that the kidnappers were Taliban because criminal gangs would have likely asked for a ransom.

His information cannot be independently verified, and US and Canadian officials still do not say for certain that the couple were abducted.

Taliban spokesman Zabihullah Mujahid told the Associated Press two months ago that no one from the group was involved.

"We do not know about these two foreigners," he said in a telephone interview.